(a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d)is not a satellite.[1]

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Contents

Pluto

The first of the dwarf planets ever discovered was Pluto. At the time, no astronomer thought to classify Pluto any differently from any other planet. Yet Pluto was always an anomaly among planets. Not only was it the smallest planet, but it was also a rocky or "terrestrial" planet in a region that ought to have only gas giants within it, according to the nebula hypothesis. Some astronomers even speculated that the Sun had captured Pluto from outside its system.

Eris

The concept of "dwarf planet" properly dates to the discovery of the scatter-disk object named Eris on January 5, 2005. With the confirmation of the identification of Eris, a formal debate began on the subject of what does, and what does not, constitute a planet. The community of astronomers decided that relying on history and tradition simply would not serve. The eight celestial bodies that now remain under the present definition of planet do not share their orbits with any other objects. The objects now called dwarf planets do not have this distinction. And yet these objects are significantly heavier than mere asteroids, in that they are so heavy that their own weight, and more particularly their own gravity, forces them to assume the round or nearly-round shape that all planets have.

Eris has another distinction that forced the debate: it is more massive even than Pluto (by twenty-seven percent) and is therefore the heaviest dwarf planet yet found. The mass of Eris is inferred from the orbital parameters, including the apsides and period, of the small body that orbits Eris. If Pluto could still be called a planet, then Eris deserved that distinction as well.

In 2006, the International Astronomical Union settled the issue. They determined that Eris was not a planet, and neither was Pluto. But the criteria they set also provoked yet another reassessment of the status of Ceres, the largest and first-discovered object in the asteroid belt. Upon consideration, the IAU declared that Ceres was a dwarf planet as well.

Today the list of dwarf planets is limited to these three: Eris, Pluto, and Ceres.